Development of Building Structural Steel with High Yield Ratio and High Yield Point Leading to Innovative Steel Structural System

NIPPON STEEL TECHNICAL REPORT No. 97 JANUARY 2008

UDC 699 . 14 . 018 . 292 : 691 . 714 Development of Building Structural Steel with High Yield Ratio and High Yield Point Leading to Innovative Steel Structural System

Takahiko SUZUKI*1 Yusuke SUZUKI*2 Yuzuru YOSHIDA*3 Shin KUBOTA*4 Yasumi SHIMURA*1 Masahiro NAGATA*1

Abstract Damage control design of building structures to absorb seismic energy using seis- mic dampers to prevent damage to beams and columns has come to be widely ap- plied to high-rise buildings. By this design method, the behavior of columns under seismic forces remains substantially within the elastic region, and naturally, the mechanical properties required for the steel used for columns should be different from those for beams. However, the same steel has been used conventionally for both the applications. In view of this, setting a target yield point higher than those of similar conventional steels, Nippon Steel has developed a new steel for building column use. This paper reports the parameters of the development, specifications and performance of the new steel.

mance items clear that steels used for building structures in the coun- 1. Introduction try. After the introduction of the New Earthquake-Resistant Design After the standardization of the SN steels, the Hanshin-Awaji Code in 1981 under the revised Building Standard Law and the qual- Earthquake in 1995 caused building owners to attach more impor- ity problems of building frames in the 1990s, the properties that steel tance to the property value of buildings, and in view of this, the re- materials for building structural use should have attracted wide at- lated industries have made efforts to enhance the seismic resistant tention. The desired performance of structural steels was studied in reliability of building structures. More recently, an inter-ministerial an effort to meet the requirements for the design and building as- project named the Development of New Building Structural System pects. Accordingly, a new grade of rolled steel for building struc- Using Innovative Materials started (under cooperation between the tural use (SN) was included in the Japanese Industrial Standards (JIS) Cabinet Office, Ministry of Economy, Trade and Industry and Min- system in 1994; the specifications for weldability (carbon equiva- istry of Land, Infrastructure and Transport) envisaging urban rede- lent Ceq) and seismic resistance (low yield ratio YR, narrow range of velopment and social capital improvement. A new building structure yield point YP, and Charpy absorbed energy vE) were newly included system, dramatically more seismic resistant and versatile than con- for the SN steels in addition to those of conventional SS (rolled steels ventional ones, called the New Structural System is being developed for general structure) and SM (rolled steels for welded structure) under the framework of the project1, 2). steels. The inclusion of SN steels in the JIS system made the perfor- The target of the New Structural System is to economically

*1 Construction & Architectural Materials Development & Engineering *3 Plate Sales Div. Service Div. *4 Osaka Sales Office *2 Steel Research Laboratories

- 71 - NIPPON STEEL TECHNICAL REPORT No. 97 JANUARY 2008 achieve an unprecedented level of seismic resistance to withstand a rise buildings (see Fig. 3) since the early 1990s, and thus, damage Scale 7 earthquake (Japanese scale) by elastic design of columns control structural design3) to reduce the extent of plastic deforma- and beams. Here, an important development policy is functional di- tions of columns and beams became a common practice. After the vision of structural members typically such as the “skeleton and infill” Hanshin-Awaji Earthquake, the need for the higher asset values and and “separate-horizontal-and-vertical-loads structure”. The philoso- prompt recovery of buildings after a heavy earthquake increased so phy of different functions for different structural members inevita- much that virtually all the structural frames of high-rise buildings bly requires new innovative structural materials, and the policy for constructed thereafter have seismic dampers. A building frame by the development of such new steel materials economically is to iden- the latest design philosophy is composed of columns, beams and tify newly required property items, improve them in priority but dras- dampers having their respective functions, and as described below, tically disregard those that are not so strongly required. Developing the steels used for them must have different mechanical properties in innovative steel materials excellent in specifically required property accordance with the functions required for each of the structural items and selectively applying them to different structural parts make members. the New Structural System viable. Dampers: Dampers are expected to deform plastically to absorb In consideration of the movement as outlined above and as a pre- seismic energy before columns and beams do. The steel cursory action for the future shift from the current building structure used for this application is required to have a low yield to the New Structural System, Nippon Steel Corporation has reviewed point, narrow range of yield point fluctuation and high the current material standards based on the philosophy of functional elongation. division of structural members and use of specialized materials for Beams: Beams are expected to deform plastically to absorb the different structural applications, and as a result, developed a new portion of seismic energy exceeding the capacity of the steel for building structural use. dampers, and to yield before columns do to prevent them from being damaged. For these purposes, the steel used 2. Elastic Column Design and Development of Heavy for beams must have a low yield ratio and narrow range Steel Plates for Column Use of yield point fluctuation. 2.1 Property items required for building structural steels A moment-resistant-frame structure composed of columns of square hollow sections and beams of H sections accounts for about 90% of steel-framed buildings. The columns and beams are con- nected to each other rigidly using diaphragms, etc., and welding is widely employed for the connection at either fabrication plants or construction sites. Under large seismic force, the columns, beam ends and column-to-beam connections are allowed to deform plastically to absorb seismic energy and thus prevent the whole structure from collapsing (see Fig. 1). Detailed structural study for low-rise build- ings under strong seismic force is omitted, as structural members are implicitly expected to deform plastically. Consequently, as shown in Fig. 2, weldability and seismic resistant properties are strongly re- quired for steel materials for building structural use regardless of which structural members they are applied to. In the meantime, a seismic energy absorbing device called a seis- Fig. 2 Performance requirement for steel used in building structure mic damper came to be incorporated in the structural frames of high-

Fig. 1 Seismic design utilizing plastic deformation capacity of beam and column Fig. 3 Damage control design preventing column damaged

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Columns: Columns support all the weight of the building to pre behavior of columns remains within the elastic deformation region, vent it from collapsing under large seismic force. Pro which meets the latest need for safety and an enhanced asset value of tected by the two-stage energy absorption by dampers a building. It is expected that such an elastic column design method and beams, columns remain substantially within an elas would not be limited only to high-rise buildings but expand to a wider tic deformation region. Since the sum of the building variety of buildings. This design method minimizes the required de- weight and seismic load imposed on a column increases formation capacity of columns, and it will be possible to relax the in lower stories, the steel used for this application must restriction on yield ratio. This will permit a higher yield point with- have a high design strength. out having to change tensile strength from that of conventional steels In consideration of these required properties, low-yield-point (see Fig. 4), and accordingly, a higher design strength without sacri- steels have been developed for damper applications and widely ac- ficing weldability. New high-yield-ratio, high-yield-point steel cepted in the market. For beam and column applications, on the other plates(BT-HT400C) for building structural use were developed based hand, in spite of significantly different plastic deformation behav- on this concept. iors required for them, the same grade of steel based on the JIS stand- Table 1 compares the specifications set out for the development ard for SN steels is still being used for both. In view of the above and of the new steel with those of Nippon Steel’s BT-HT325 and 355 expecting that the damage control structural design will become (TMCP steels for building structural use), which have design strengths widely practiced, the authors decided to propose a new steel grade higher than that of the SN steels, and SA440; all the comparative different from the SN steels and having the characteristics and func- steels have yield ratios of 80% or less. The range of yield ratio of the tions required for the columns in the damage control structural de- developed steel was expanded (relaxed) to 90% or less, and its de- sign. sign strength was set at as high as 400 N/mm2, which is higher than 2.2 Proposal of new steel grade for building column application that of BT-HT325 by 23% and close to that of SA440, while its ten- The height and column-to-column span of buildings are increas- sile strength was in the same class as that of BT-HT325 (490 N/ ing, and as a result, increasingly thicker steel plates came to be used mm2). for building columns. Since increase in material thickness leads to The chemical composition specified for the new steel was the increased structural weight and welding work, use of high-design- same as that for BT-HT325, and indicators of weldability, namely strength steels (for example, SA440, high-performance steel for build- the carbon equivalent (Ceq) and chemical composition on sensitivity 2 ing structural use of a 590-N/mm class) is effective in controlling of welding crack (PCM), were set lower than those of BT-HT355 and the increase in construction costs. It should be noted, however, that SA440. As a result, plates of the new steel do not require preheating to obtain a high design strength of a steel while keeping its yield for normal welding in all the thickness range. In addition, since the ratio at 80% or less, it is necessary to increase its tensile strength as developed steel was meant mainly for application to welded square well, which means increased addition of carbon and other alloying hollow section columns, the upper limits of P and S were set lower, elements. However, increased use of alloying elements is detrimen- and the specification includes provisions for the homogeneity of tal to weldability, causing significant increase in hardness and crack- material properties in the thickness direction and ultrasonic test, like ing of heat-affected zones (HAZs) of weld joints. All these mean that, even if a low yield ratio is realized with a high-strength steel, it will not be able to fully exert its plastic deformation capacity be- cause of easy occurrence of brittle fracture starting from a hardened or cracked portion. To prevent this, stringent work control such as preheating is re- quired for welding of SA440. As described above, design strength, yield ratio and weldability, in close interrelation with each other, exert influence on the properties of structural steels. These three prop- erty items are in a good balance in the SN steels, but when a design strength higher than that of the SN steels is attempted, the good bal- ance between the three is difficult to maintain, and it becomes neces- sary to choose one of the two mutually incompatible property items, low yield ratio and high weldability. To prevent a building from being destroyed by a strong earth- quake and minimize the residual deformation of the whole structure to enable its reuse, it is essential to take design measures so that the Fig. 4 Development concept of high yield point steel

Table 1 Comparison of specifications with conventional steel

Standards Design strength Thickness Yield point Tensile strength Yield ratio Ceq VE 0 fHAZ (N/mm2) (mm) (N/mm2) (N/mm2) (%) (%) (J) (%) BT-HT400C 400 19-100 400-550 490-640 ≦90 ≦0.40 70≦ ≦0.58 BT-HT325 325 40-100 325-445 490-610 ≦80 ≦0.40 27≦ - BT-HT355 355 40-100 355-475 520-640 ≦80 ≦0.42 27≦ - SA440 440 19-100 440-540 590-740 ≦80 ≦0.47 47≦ -

- 73 - NIPPON STEEL TECHNICAL REPORT No. 97 JANUARY 2008 in the case of Class C of the SN steels. that of BT-HT325. The TMCP was originally developed to increase As regards to impact properties, assuming the standard welding steel strength by forming a fine metallographic structure, but when condition of CO2 gas-shielded arc welding (CO2 welding), the chemi- applied to steels for construction use, this advantage of the process is cal composition (more specifically, fHAZ, an index of the toughness abandoned to lower the yield point elaborately so as to obtain a yield of HAZs by MAG welding4)) was so defined that the Charpy ab- ratio of 80% or less. The developed steel, in contrast, was produced sorbed energy of a HAZ at 0℃ was 70 J or more. This is because, to applying the TMCP according to its original purpose to obtain a high prevent brittle fracture at a welded beam-end joint designed to de- yield point, which made it possible to raise the design strength of the form plastically, it is desirable to secure a Charpy absorbed energy plates at production costs comparable to those for conventional steels. of 70 J or more at 0℃5). In consideration of the risk of welded joints Photo 1 compares the microstructure of the new steel with that of building pieces and other fixtures serving as initial points of low- of conventional steel for building structural use. The crystal grains stress brittle fracture and the fact that welding work at construction of the developed steel are fine, which raises the yield point and en- sites is not always done under due control, it is important for steel hances impact properties of the base metal. The authors tested the for construction use to have good weldability and impact properties. material properties of the test-produced plates 25, 50 and 100 mm in The plate thickness of the developed steel was set at 19 to 100 thickness; the results are described below. mm to make it possible to fabricate columns from the lowest floor to Table 2 shows the chemical composition of the specimen plates the building top using the same grade and class of steel. The devel- together with their values of Ceq, PCM and fHAZ, and Table 3 the re- opment policy assumed that columns would be designed not to de- sults of their mechanical test. Note that the plates 25 and 50 mm in form plastically up to an ultimate condition, and for this reason, the thickness were rolled from the same heat of molten steel. allowable ranges of yield point and tensile strength were set wider The stress-strain relationship at tensile test is shown in Fig. 5. than those for conventional steels. The yield point and yield plateau tend to become unclear with in- 2.3 Production of plates of high-yield-ratio, high-yield-point steel creasing plate thickness, and the stress-strain curve assumes a round- Plates were test produced in accordance with the specifications roof shape; the tendency is the same with conventional TMCP steels. for the developed steel and their material properties were examined. When yield point does not show clearly, 0.2% proof stress is used The thermo mechanical control process (TMCP) was applied to the instead for guaranteeing the standard value. The yield ratio of the plate production, and a high yield point was obtained through accel- 25-mm thick plate was over 80% and those of the 50- and 100-mm erated cooling even though the chemical composition was the same thick plates were less than 80%. as that of a 490-N/mm2 class steel; a yield point of 400 N/mm2, nearly Fig. 6 demonstrates the energy transition curves at Charpy im- that of SA440, was obtained while securing the same weldability as pact test. The transition temperature ranged from Ð74 to Ð52℃, and

Photo 1 Micro-structure of conventional TMCP steel and new steel

Table 2 Chemical compositions of sample steel (mass %)

New steel Thickness (mm) C Si Mn P S Ceq PCM fHAZ Specification 19-100 ≦0.20 ≦0.55 ≦2.00 ≦0.020 ≦0.008 ≦0.40 ≦0.26 ≦0.58 Sample A 25 0.14 0.27 1.30 0.014 0.003 0.37 0.22 0.39 Sample B 50 0.14 0.27 1.30 0.014 0.003 0.37 0.22 0.39 Sample C 100 0.10 0.22 1.54 0.007 0.002 0.37 0.19 0.35

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Table 3 Mechanical properties of sample steel

Tensile test Charpy impact test Through thickness tensile test Thickness Tensile Absorbed energy Yield point strength Elongation Yield ratio at 0℃ Reduction of area (mm) (N/mm2) (N/mm2) (%) (%) (J) (%) 21≦ *1 25≦ (ave.) Specification 19-100 400-550 490-640 ≦90 70≦ 23≦ *2 15≦ (each) Sample A 25 455 554 25 *1 82 311, 305, 315 (ave.310) 67, 62, 72 (ave.67) Sample B 50 442 576 26 *1 77 316, 317, 321 (ave.318) 71, 71, 71 (ave.71) Sample C 100 412 546 34 *2 76 327, 339, 337 (ave.334) 80, 78, 78 (ave.79) *1: Gage length = 200mm *2: Gage length = 50mm

Fig. 5 Stress-strain curves from tensile test Fig. 7 Hardness distribution in direction of thickness in section

at a test temperature of 0℃, all the readings of the specimens were on the upper shelf always exceeding 300 J, which evidences excel- lent impact properties of the developed steel due to the fine crystal structure as seen with Photo 1, better than those of conventional TMCP steels of low yield ratios. Fig. 7 shows the results of sectional hardness test in Vickers hard- ness. The hardness fluctuation within a section surface increased with increasing plate thickness, the reading being higher nearer the plate surfaces. Substantially the same tendency is seen with conventional TMCP steels, and it does not adversely affect the structural perfor- mance of the material. To confirm the weldability, the authors conducted maximum HAZ hardness test (JIS Z 3101) and y-groove weld cracking test (JIS Z Fig. 6 Energy transition curves from Charpy impact test 3158) using the 50-mm thick plates and by CO2 welding. The test conditions and results are given in Tables 4 and 5. The maximum

Table 4 Test result of maximum hardness test in weld heat-affected zone Thickness Test method Welding process Welding material Pre-heat Heat input HV10 JIS Z 3312 50mm JIS Z 3101 CO *1 None 17 kJ/cm 253 2 YGW18

*1: CO2 gas-shielded arc welding

Table 5 Test result of y-groove weld cracking test

Thickness Test method Welding process Welding material Pre-heat Heat input Surface crack Section crack Root crack JIS Z 3312 50mm JIS Z 3158 CO None 17 kJ/cm None None None 2 YGW18

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HAZ hardness (HV10) was 253 and no undesirable hardness change was observed. No cracking was observed in the y-groove weld crack- ing test, either, attesting to excellent weldability.

3. Performance of Building Columns of Welded Square Hollow Sections Using Plates of High- yield-ratio, High-yield-point Steel 3.1 Confirmation of performance of welded joints through weld- Fig. 8 Groove configuration of welded joints ing test 3.1.1 Outlines of test Since building columns of welded square hollow sections are inner diaphragms, and as a result, the heat input was nearly 750 kJ/ mostly fabricated employing a large-heat-input welding method such cm. In the case of SAW, skin plates 50 mm in thickness were welded in one pass by single-layer welding using two electrodes. as CO2 welding, electro-slag welding (ESW) or submerged arc weld- ing (SAW), the authors prepared actual-size joints of the 50-mm thick The welding consumables were selected in consideration of the plates by each of the three methods and tested the mechanical prop- target performance of the welded joints. To allow a relaxed welding erties of the joints and HAZs. condition and secure a weld metal toughness of 70 J, YGW18 (JIS Z

Table 6 shows the welded joint performance targeted at the test. 3312) was selected for CO2 welding. In consideration of the decrease The target minimum tensile strength of the welded joints and weld in yield strength of weld metal under a large heat input, S582-H (JIS metal was set at 490 N/mm2, equal to the tensile strength of the base Z 3183) of the 590-N/mm2 class was selected for SAW. For ESW, metal. Assuming that the yield strength of the weld metal at a corner YES51 (JIS Z 3353), conventionally used for 490-N/mm2 class steels, joint of the square hollow section by SAW and that of a column-to- was selected because it was enough for welded joints to have the same yield strength as that of beams. No preheating was done in any column joint by CO2 welding should be equal to or higher than that of the base metal, the target yield strength of the weld metal of these of the welding methods. joints was set at 400 N/mm2. On the other hand, with respect to the 3.1.3 Test results inner diaphragm welding by ESW, supposing that the material of Fig. 9 shows the notch positions of Charpy impact test pieces, beams would be of the TMCP325 class and that it would be enough Fig. 10 photographs of macrostructures and hardness distribution for a joint to have a yield strength equal to that of the beam, the graphs of the welded joints, and Table 8 the results of their tensile target yield strength of the weld metal was set at 325 N/mm2. The and Charpy impact tests. The macro photographs attest that there are target value of Charpy absorbed energy at 0℃ (notch toughness) no welding defects and the welded joints had a sufficient width and good shape of penetration. was set at 70 J for CO2 welding, corresponding to fHAZ of the base metal, and those for large-heat-input SAW and ESW were set at 27 J, The tensile test of the welded joints was conducted using uni- equal to the notch toughness of conventional TMCP steels. The tar- form gauge test pieces prepared by removing excess weld metal and get maximum hardness of HAZs was set at 350. backing plate by machining, and the same of the weld metal using 3.1.2 Welding conditions round bar test pieces cut out from the 1/4 thickness position. All the The welding conditions for the test were defined such that the test pieces cleared the target values. All the tensile test pieces of the heat input would be as large as possible within the range of common welded joints failed at a HAZ of the base metal, which coincided steel structure fabrication. Fig. 8 shows groove configurations, and with the softened portion explained herein later. Table 7 the condition for each of the welding methods. In the case of In the case of the diaphragms welded by ESW, hardness was diaphragm welding by ESW, plates of the same steel grade and thick- measured at the thickness center, and in the cases of plates welded ness (50 mm) as those of the column skin plates were used for the by SAW and CO2 welding, at 2 mm below the plate surface. While

Table 6 Targeted level of welded joint performance

Welding Yield strength Tensile strength Tensile strength Notch Maximum process of weld metal of weld metal of welded joint toughness*3 hardness Inner diaphragm ESW*1 325≦ 490≦ 490≦ 27≦ ≦350 Corner joint SAW*2 400≦ 490≦ 490≦ 27≦ ≦350

Column to column CO2 400≦ 490≦ 490≦ 70≦ ≦350 *1: Electroslag welding *2: Submerged arc welding *3: Charpy absorbed energy

Table 7 Welding conditions Current Voltage Welding speed Heat input Welding process Welding consumable Welding pass (A) (V) (cm/min) (kJ/cm) ESW JIS Z 3353 YES51 1 380 52 1.6 741 SAW JIS Z 3183 S582-H 1 1900/1500*1 40/45*1 18 485

CO2 JIS Z 3312 YGW18 1-44 230-280 28-32 19-65 ≦40 *1: Twin electrode leading electrode/trailing electrode

- 76 - NIPPON STEEL TECHNICAL REPORT No. 97 JANUARY 2008 the HAZs of SAW and ESW exhibited a tendency to soften, and in toughness of HAZs tended to lower at the Charpy test with higher contrast, those of CO2 welding another to harden, all the readings welding heat input. The column-to-column joints by CO2 welding satisfied the target hardness of HV10 ≦ 350. Judging from this and exhibited absorbed energy values significantly above the target value the fact that all the welded joints cleared the target values for the of 70 J corresponding to the specification of fHAZ. The Charpy ab- tensile test without showing any sign of brittle fracture, the change sorbed energy of all the SAW joints cleared the target value of 27 J in hardness will not adversely affect the structural performance of although some of the readings were lower than the specification value the steel frame. A similar tendency has been observed with conven- for the base metal of 70 J. The absorbed energy tended to be lower at tional TMCP steels as well. positions away from a fusion line (FL); this is presumably because As is the case with conventional building structural steels, the the positions of these test pieces were nearer the thickness center and they were affected by center segregation. The absorbed energy of ESW joints was roughly 70 J when the heat input was 740 kJ/cm, and the amount of penetration under this condition was roughly 3 mm on one side. However, one must be careful about increasing heat input to secure sufficient penetration because, if excessive, it leads to lowering of HAZ toughness. It should be noted that the above Charpy test results of the SAW and ESW joints are substantially at the same level as those of con- ventional steels. There are still various discussions about how good impact properties a welded joint must have to withstand possible destructive force and further clarification of the issue is awaited. While Fig. 9 Location of Charpy impact test specimens measures from the design and construction viewpoints are important for preventing destruction of structures, application of technologies

Fig. 10 Macro-appearance and hardness test results of welded joints

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Table 8 Results of tensile tests and impact tests Test of weld metal Test of welded joint V-notch Charpy impact test Welding process Yield Tensile Tensile Location of Absorbed energy (J) strength*1 strength*1 strength*1 fracture (average of three tests) Depo*2 70 F.L.+1mm 89 364 530 ESW - - Depo*3 184 F.L.+3mm 110 362 526 F.L. 140 F.L.+5mm 145 F.L.+1mm 61 472 674 546 Base metal Depo*2 97 SAW F.L.+3mm 57 480 678 553 Base metal F.L. 71 F.L.+5mm 35 653 701 603 Base metal Depo 139 CO F.L.+1mm 240 2 682 696 613 Base metal F.L. 198 *1: N/mm2 *2: Center of weld metal *3: Edge of weld metal to improve HAZ toughness (typically such as HTUFF¨) 6) is effec- plied evenly onto its upper end face, and the axial deformation of the tive from the viewpoint of materials. specimen was measured using displacement gauges provided between 3.2 Confirmation of column performance through compression Test Table 10 Mechanical properties of sample steel using structural experiment While columns are expected to behave within an elastic region under foreseeable external force, it is necessary to confirm how they Thickness Yield point Tensile strength Elongation*1 Yield ratio fail ultimately under force larger than foreseeable. The deformation (mm) (N/mm2) (N/mm2) (%) (%) capacity of a structural member decreases with increasing yield ra- 12.4 439 538 23 82 tio, and for this reason, when steel with a high yield ratio is used for building columns, it becomes important to ensure their elastic be- 11.1 439 531 23 83 havior through measures such as keeping the column overdesign fac- 9.1 437 527 24 83 tor within a prescribed range. *1: Gage length = 200mm With a structural member under permanent compressive axial force such as a building internal column, however, stress fluctuates mainly in the compressive range, and the member finally fails by local buckling. The occurrence of local buckling is influenced con- siderably by width-thickness ratio; actually some papers report that the level of yield ratio does not significantly affect deformation per- formance7). The authors confirmed through tests of the developed steel that, in the case where maximum strength was determined ulti- mately by local buckling, it was possible to expect a steel having a high yield ratio and yield point to bear in the same manner as that of conventional low-yield-ratio steels. Fig. 11 Set-up of stub-column test 3.2.1 Stub-column test To clarify the relationship between the local buckling behavior and yield ratio of a square hollow section, the authors conducted stub-column test. Tables 9 and 10 show the chemical composition and mechanical properties of the steel used for the test. Note that, because of the limited capacity of the test facility, the 12-mm thick plates, outside of the BT-HT400C specification, were prepared and used. Five column specimens were fabricated using plates of the same thickness; the width-thickness ratio was changed by changing the outer dimension of the specimens, and their height was set at three times the outer dimension. The corner joints were welded by full- penetration CO2 welding, and the excess weld metal was removed after the welding work. As shown in Fig. 11, a specimen was placed between the loading and bearing plates such that the load was ap- Fig. 12 Stress-strain curves obtained from stub-column test

Table 9 Chemical compositions of sample steel using structural experiment (mass%)

Thickness C Si Mn P S Ceq PCM fHAZ 12mm 0.12 0.24 1.41 0.010 0.002 0.37 0.20 0.37

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Table 11 Results of stub-column test applied in two cycles; then positive and negative displacements twice

δp were applied in two cycles, δp being the deformation under the σmax εmax τ= μ= D/t full plastic moment; then 4 times δ , 6 times δ , and so forth until (N/mm2) (%) σ /σ ε /ε p p max y max y the specimen failed. No.1 9.6 655 10.50 1.57 51.8 The specimens underwent local buckling and recorded the maxi- No.2 17.4 480 2.71 1.15 13.3 mum moment strength as follows: specimen No. 1 during the load No.3 24.1 426 0.69 1.02 3.4 shift from Ð 4 times δp to + 6 times δp ; specimens Nos. 2 and 3 during the cycle of ± twice δp ; and specimen No. 4 when the load- No.4 27.0 422 0.55 1.01 2.7 ing was nearly + twice δp . Fig. 14 shows the relationship between No.5 34.7 410 0.28 0.98 1.4 the bending moment and rotation angle of the specimens. The bend- D, t : Width and thickness of stub-column specimen ing moment was calculated in consideration of the additional bend- ing resulting from the compressive axial force. The test results are σmax : Maximum strength obtained from stub-column test listed in Table 13, where M is the maximum moment strength, εmax : Strain atσmax obtained from stub-column test max η

σy : Yield strength obtained from tensile test is the cumulative plastic deformation factor, and θmax is the defor- ε : =σ /E (E : Young’s modulus) y y mation under Mmax calculated from a skeleton curve. 3.2.3 Discussion on test results the loading and bearing plates. The authors compared the results of the stub-column and beam- Fig. 12 shows the stress-strain relationship thus obtained, and column tests described above with those of similar tests using speci- 2 Table 11 the test results. The symbol σmax indicates the maximum mens of conventional steels (400 to 590-N/mm class steels with 7) stress, and εmax the strain under σmax. All the specimens recorded yield ratios of 70 to 90%) . Here, the results of the stub-column test decrease of the strength when local buckling occurred; specimen No. were compared in terms of the strain-plasticity ratio calculated by 1, which had the smallest width-thickness ratio, buckled in an accor- dividing the strain under the maximum stress by yield strain, and the dion fashion, and exhibited higher strain and stress than the others results of the beam-column test in terms of the ductility factor η of did. the skeleton curve by δp. In consideration of different strengths of 3.2.2 Beam-column test In addition to the above, using the equipment shown in Fig. 13, the authors conducted beam-column test, wherein cyclic shear force Table 12 List of beam-column specimens was applied to the center of a column specimen kept under constant, D t L D/t N/N compressive axial force. Four specimens with width-thickness ra- (mm) (mm) (mm) y tios D/t ranging from 18 to 36 were prepared, and the axial force ratio (ratio of the force imposed on the specimen to its yield axial No.1 216 12.4 17.5 1 400 0.3 force, which is yield point × sectional area) was set at 0.3. The length No.2 300 12.4 24.4 2 000 0.3 of the specimens was so defined that their slenderness ratio was No.3 308 11.1 27.7 2 000 0.3 roughly 20. A loading plate was provided at the length center of each No.4 324 9.1 35.7 2 000 0.3 specimen in the form of a through diaphragm. Table 12 shows the specifications of the specimens; the 11.1- and 9.1-mm thick plates L : Length of stub-column specimen were prepared by machining the 12.4-mm thick plates from both N : Applied compression load N : =σ A ( A: section area ) sides. The cyclic shear loads were applied in the following manner: y y first, positive and negative loads one half the yield strength were

Fig. 13 Set-up of beam-column test

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No.1 No.2

No.3 No.4

Fig. 14 Results of beam-column test

Table 13 Results of stub-column test tional steels demonstrate substantially the same tendency, which in-

Mmax θmax μ= dicates that the deformation behavior of the developed steel under (kN¥m) (rad) θ /θ compressive force can be evaluated using substantially the same max p method as that for evaluating the conventional low-yield-ratio steels. No.1 391 0.05 6.6 No.2 693 0.02 3.1 4. Summary No.3 619 0.02 3.2 As a result of recent use of seismic dampers and introduction of a design method whereby building beams are made to yield before No.4 508 0.01 1.6 columns do, columns are made to remain in an elastic region even M : Maximum moment strength obtained from beam column test max under strong seismic force. In consideration of this recent trend, the θ : Rotation angle at M on skeleton curve obtained from beam max max authors initiated the idea of a new high-yield-point and high-yield- column test ratio steel for building column application giving due consideration θp : Elastic rotation angle corresponding to Mp M : Full plastic moment of specimen considering axial load to higher design strength and improved weldability, test produced p heavy plates of the new steel, fabricated welded square hollow sec- tions and confirmed the performance of the developed steel when different steels, the comparison was done in terms of the equivalent applied to building columns. width-thickness ratio (D/t)eq expressed by equation (1). Fig. 15 shows As the design strength of building structural steel increases, it is the relationship between the equivalent width-thickness ratio and possible to restrict the increase in the skin plate thickness of square- ductility factor. hollow-section columns and decrease steel weight and welding vol- ume. In addition, higher design strength expands the region where D / t = D / t / E eq σ y (1) building columns behave elastically and increases the rigidity ratio The test results of conventional steels indicate the following: between seismic dampers and the main building frame, which makes buckling behavior depends mainly on the width-thickness ratio; when it easier for the dampers to show their effect. In designing the chem- the ratio is large, buckling occurs soon after yielding, which leads to istry of the new steel, the authors allowed a higher yield ratio than out-of-plane deformation before the stress of the steel material in- those of conventional structural steels and aimed at enhancing creases, and as a result, buckling behavior changes only slightly de- weldability, and as a result it became possible to lower the costs for pending on yield ratio; and only when the width-thickness ratio is welding work and improve the quality and performance of welded small, the ductility factor tends to increase slightly as the yield ratio joints. Another advantage of the developed steel is that, depending decreases. It is clear from Fig. 15 that the developed and conven- on heat input and inter-pass temperature, it can be welded using com-

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Fig. 15 Relationships between ductility and width-thickness ratio mon welding consumables for 490-N/mm2 class steels. elastic region and thus to enhance the seismic resistance of build- When applying the developed steel, it is essential to confirm that ings. It is expected that the developed steel, together with the build- the columns will behave substantially within an elastic region under ing design philosophy of elastic columns, is widely applied, and its the design conditions. More specifically, this will be done, directly, application spreads from high-rise buildings to middle- to low-rise by seismic response analysis or, more indirectly, by securing a col- buildings to enhance the reliability and safety of steel structure build- umn overdesign factor that would guarantee a overall collapse mode. ings. On the other hand, one may point out the possibility of the be- havior of building columns going far beyond an elastic region and References becoming plastic because of things such as unexpectedly large seis- 1) Joint Committee on Research and Development of New Building Struc- tural System: Research and Development Project of New Building Struc- mic force, fluctuation of material properties and inadvertencies in tural System Using Innovative Materials. JSSC. (59), 25 (2006) design and erection work. A common response to this question would 2) Committee on Research and Development of New Building Structural be that the plastic deformation capacity of the structural steel mate- System: Research and Development of New Building Structural System. rials would cope with such unforeseeable disturbances. However, ANUHT. (41), 1 (2006) with increasing steel strength, the risk of brittle fracture increases 3) Iwata, Huang, Kawai, Wada: Study on Damage-tolerant Structures. AIJ J. Technol. Des. (1), 82 (1995) and the relied-upon plastic deformation capacity may disappear. 4) JISF: Market Development Committee Regulation, “Rolled Steels for Another approach is to cope with such disturbances by lowering the Welded Seismic Building Structure”. 2003 level of stress that may be imposed on structural members without 5) Building Center of Japan: Guidelines for Preventing Brittle Fracture in relying on their plastic deformation capacity. This might be effective Beam-end Welding Connections. 2003 6) Kojima et al.: Development of High-HAZ-toughness Steel Plates for Box especially in designing columns, which bear the whole weight of a Columns with High-heat-input Welding. Shinnittestu Giho. (380), 33 building. (2004) The new steel was developed not only to reduce the costs for 7) Kadono et al.: Experimental Study on the Effect of Yield Ratio on the fabrication and construction of building frames but also to promote Bending Strength Increasing Ratio and the Ductility of Steel Structures’ the building design philosophy to use columns exclusively in the members. AIJ J. Struct. Constr. Eng. (40B), 673 (1994)

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